Apparatus for electrically testing the thickness of thin layers



June 1, 1943. H, TNT ET 2,320,761

APPARATUS FOR ELECTRICAINJLY TESTING THE THICKNESS OF THIN LAYERS V Filed Oct. 22, 1940 2 Sheets-Sheet 1' r I B Ch /Mend June 1, 1943.- T I H T ETAL, 2,320,761

APPARATUS FOR ELECTRICALLY TESTING THE THICKNESS OF THIN LAYERS Filed Oct. 194;) 2 Sheets-Sheet 2 figs Fly? E I lawns 5 i fw Fatented June 1, 1943 APPARATUS FOR ELECTRICALLY TESTING THE THICKNESS OF THIN LAYERS 4 William Henry Tait, Pinncr, and Bruce Chalmers; London, England Application October 22, 1940, Serial at. 3 2,290 1 e In Great Britain October 26, 1939 8 Claims. (01. 175-182) This invention relates to electro-magnetic apparatus for measuring the thickness of thin layers, of the kind in which such thickness is measured by measuringchanges in a magnetic circuit formed by a transformer cor and a magnetic base on which is supported the layer whose thickness is to be measured and which thusforms a gap" in the magnetic circuit. The property of the magnetic circuit that depends upon the thickness of the layer may be the self-inductance or the impedance of a coil surrounding part of the magnetic circuit or it'may be the mutual inductance of two such coils. Measurements made with such apparatus are accurate only for layers on basis metals which have the same magnetic properties and whose shapeand size lie within certain limits. Various methods have been and side view of one convenient form of transformer core, and

Figures '1 to 10 show tour methoclsof using a three-limbed transformer core.

In carrying our invention into effect in one convenient manner as, for example, in its ap- I plication to the measurement of the thickness of tin coating I on tin-plate, and as illustratedin Figure. 1, we-form our apparatus-with a U- shaped core a of magnetic material such as iron, upon one limb of which isa primary coil b to which a standard alternating current is approposed to allow corrections to be .made for variations in the magnetic properties and dimensions and profile of the magnetic base, but all such methods are dependent on the assumption that the magnetic base is' flat and uniform over its surface and that its properties are non-directional.

'We have found, however, that these conditions are not always satisfied, and an object of the present invention is to devise simple and practlcal apparatus by which accurate correction can be made for wide variations in the dimensions and magnetic properties of the magnetic base and by which the effect of curvature of thebase can be" minimised.

plied, while the other limb carries a' secondary coil -c connected to a galvanometer d orother suitable current measuring device. We arrange to saturate a part of the-core by passing direct current as from a'battery in Fig, 1a or rectifier,

as in Fig. 1 through an additional coil e encircling part of the core (for example that part midway between the input and output coils) or alternatively, as shown in Fig. 2 we may use-the j current passed by a half wave rectifier for both saturating current and primary current simulta- .neously. Or, alternatively as in Figure 20 we The invention includes means for compensatproperties or the base, this measurement being uninfluenced by variations in the thickness of layer. t v

The invention also consists in apparatus of the kind referred to in which each pole-piece ofthe transformer core is shaped approximately to the jorm of a part of an ellipsoid oi whichthe major axis may be inclined to the vertical.

Further features or the invention will be aparent from the description given hereafter. The accompanying drawings illustrate several modes of carrying out the invention.

Figures 1, la, 2, 2a, 3 and 4 are diagrams i ustrating six convenient circuit arrangement in accordance with the invention.

Figures 5 and 6 are respectively a front view ance with the invention can therefore. be made tomeasure to within narrow limits the magneticmay superimpose a direct current, such as from an accumulator upon the alternating current through the primary coil. We may obtain the rectified current by the use of a rectifier e which must be of V2 wave type in thecircuits shown in Figures 2: 3 and 4, but in other cases may be of /2 or full wave type.

When a reading of the magnetic properties of the base material has been taken the rectified or direct current can be discontinued and the instrument used in the'known manner to measure .coating thickness, correction for magnetic properties being applied in any approved manneras by means of a rheostat I with calibrated sliding contact in the circuit. We may embody With the arrangements described the current changes in the secondary may vary over. wide range and it the indicating instrument has to cover the whole range the useful part might be confinedto a small part of the instrument scale,

allowing insuilicient accuracy or reading. We

therefore prefer to provide a current opposed to the secondary so as to suppress the true zero so that the indicating instrument only starts to read from its zero mark when a certain current is already flowing in the secondary output circuit of the measuring transformer. Figure 3 shows an arrangement of the circuit in which a single transformer h providing the backing-oil current serves for both the check and test readings. In Figure 4. we provide twotransformers,-

-asshownin Figuresand6. It will be understood that while we have deh for the test reading and i for the check read ing. This last arrangement is preferred for high sensitivity circuits.

A further advantage of the use of backing-oi! currents is that by suitable adjustment of ,windings and magnetic gaps it is possible to make the instrument, to a considerable extent, independent of fluctuations of voltage in the electric mains. 1

As an alternative to the U-shaped core we may use a core with three parallel limbs magnetically linked at one end, the primary coil encircling one form-each of the pole-pieces of the core so that it approximates to part of an ellipsoid of which the major axis may be inclined to thevertical,

scribed the invention, in its application to the measurement of the thickness of the coating upon 1 tin-plate, it may be applied with equal eflect and like advantage to the measurement of the thickness of any thin layer or lamina which'may be made to serve as a gap in the magnetic circuit associated with means for rneasuring changes in the magnetic circuit.

We claim: 1 1. Apparatus for determining the thickness of a relatively thin layer carried by a magnetizable base member comprising, a transformer including a primary winding and a secondarywinding and a magnetizable core adapted torbe bridged at ends thereof by said base member forming a magnetic circuit with said layer forming a gap in the magnetic circuit, means for supplying alternating current to said primary winding, means for measuring the voltage across the secondary winding, and means for magnetically saturating said core.

2. Apparatus for measuring'the thickness of relatively thin layers each carried by a different magnetizablebase member comprising, a transformer having a primary winding and a sec-- ondary winding and a magnetizable core adapted to be bridged at ends thereof by one of the base members so as to form a magnetic circuit with a layer to be measured forming a gap in the magnetic circuit, means for impressing alternating current across said primary winding, means for measuring the current flowing in the secondary winding, and means for magnetically saturating said core'tocompensate variations in the magnetic characteristics of' the base members.

3. Apparatus for measuring the thickness of a relatively thin layer carried by a magnetizable base membercomp'rising, a magnetizable core forming a part of a magnetic circuit which includes the base member with the layer to be measured in said ma netic circuit, a primary winding on said core, means for supplying-alternating current to said primary winding, 8; secondary winding on said core, means for measuring the current flowing in said secondary wind ing, and means operable during apart of acycle of the alternating current for magnetically saturating said core to compensate variations in the magnetic characteristics ofsaid base member.

4. Apparatus for measuring the thickness of a relatively thin layer carried by a magnetizable base membercomprising, a magnetizable core forming a part of a magnetic circuit which includes the base member with the layer to be measured in said magnetic circuit, a primary winding on said core, means for supplying alternating current to said primary winding, a secondary winding on said core, means for measuring the current flowing in said secondary winding, and means including a source of direct current for magnetically saturating said core.

5. Apparatus for measuring the thickness of a relatively thin layer carried by a magnetizable base member comprising, a magnetizable core forming a -part of a magnetic circuit which includes the base member with the layer to be measured in said magnetic. circuit, a primary winding on said core, means for supplying alternating current to said'primary winding, a secondary winding on said core, means for measuring the current flowing in said secondary winding, a supplementary coil on said core, and means for energizing the supplementary coil to magnetically saturate said core.

6. Apparatus for measuring the thickness of .a relatively thin layer carried by a magnetizable base member comprising, a magnetizable core forming 'a part of a magnetic circuit which includes the basemember with the layer to be measured in, said magnetic circuit, a primary windingon said core, means for supplying alternating current to said primary winding, a secondarywinding on said core, means for measuring the current flowingin said secondary'winding, a supplementary coil arranged onsaid cor'e between the primary and secondary windings, and'means for supplying direct current to said supplementary coil for magnetically saturating said core.

7. Apparatus for measuring the thickness of a relatively thin layer carried by a magnetizable base member comprising, a magnetizable core forming a part -'of a magnetic circuit which includes the base member with the layer to be measured in said magnetic circuit, a primary winding on said core, means for supplying alternating current to said primary winding, a secondary winding on said core, means for measuring the current flowing iii said secondary winding, and means for impressing direct current across said primary winding.

8. Apparatus for measuring the thickness of a relatively thin layer supported on a magnetizable base member comprising, a U-shapedmagnetizable core adapted to be bridged by the base member with the layer to be measured between the ends of the core and the base member, a

- primary windin'g on said core, means for supply- WILLIAM HENRY TAIT. "BRUCE CI-IALMERS'. 

